1. Field of the Invention
The present invention relates to an image pickup lens and particularly, to an image pickup lens of a single-piece construction, which is used in an image pick-up device e.g., a CCD camera, utilizing an image pickup element such as CCD, CMOS and the like mounted on a portable computer, a visual telephone, a portable telephone and the like, and which can be reduced in size and weight.
2. Description of the Related Art
In recent years, multimedia have been developed remarkably, and for example, the demand for a camera utilizing image pickup element such as CCD, CMOS and the like, e.g., a CCD camera mounted on a portable computer, a visual telephone, a portable telephone and the like, is being increased remarkably. Such a CCD camera is required to be mounted in a limited space and for this reason, it is desired that the CCD camera is small in size and lightweight.
Therefore, it is also desired that an image pickup lens used in such a CCD camera is likewise small in size and lightweight.
A so-called single lens made using a single lens piece is conventionally used as such an image pickup lens.
FIG. 1 shows such a conventional image pickup lens of a single-piece type. The lens includes a lens body 10 constituted by a positive lens. A diaphragm 11 is disposed at a location displaced from the lens body 10 toward an object. A first cover glass 12, a second cover glass 13 and an image pickup surface 14 which is a light-receiving surface of an image pickup element such as CCD or CMOS, are disposed sequentially at locations displaced from the lens body 10 toward an image surface. Lens faces of the lens body 10 are called a first face and a second face disposed sequentially in the named order from the side of the object.
The image pickup lens is set under the following conditions:
f=4.10 mm; F=2.2; 2xcfx89=60.0; Petzval sum=0.681
wherein f represents a maximum focal length; F represents an F-number; and 2xcfx89 represents an angle of view. If an X-axis is taken in a direction of an optical axis, and an z-axis is taken in a direction perpendicular to the optical axis, and if a direction of travel of light is defined to be positive, the aspherical shape of the lens is represented by the following equation:   Z  =                              x          2                r                    1        +                              1            -                                          (                                  k                  +                  1                                )                            ⁢                                                x                  2                                                  r                  2                                                                          +          a      ⁢              xe2x80x83            ⁢              x        4              +          b      ⁢              xe2x80x83            ⁢              x        6            
wherein each of k, a and b is an aspherical factor.
However, such conventional image pickup lens suffers from a problem that a good image surface cannot be provided, because of a large Petzval sum and a large image surface curvature. In addition, various aberrations such as a longitudinal chromatic aberration, a chromatic aberration of magnification, a spherical aberration, an astigmatism and the like are large and cannot be corrected appropriately and hence, good optical characteristics cannot be provided.
FIG. 2 shows chromatic aberrations of magnification in light ray courses A, B and C in the image pickup lens shown in FIG. 1. It can be seen from FIG. 2 that each of the chromatic aberrations of magnification is large and particularly, is large in a region around the image pickup lens, thereby bringing degradations in lens characteristics. A spherical aberration, an astigmatism and a distortion in the image pickup lens are shown in FIG. 3. It can be seen from FIG. 3 that all of the spherical aberration, the astigmatism and the distortion are large and particularly, the spherical aberration and the astigmatism are large and hence, sufficient optical characteristics cannot be provided.
Accordingly, it is an object of the present invention to provide an image pickup lens, which can be reduced in size and whose optical characteristics can be enhanced remarkably in a simple structure.
To achieve the above object, according to the present invention, there is provided an image pickup lens including a lens body, wherein at least one of faces of the lens body is formed into an aspherical shape, and at least one of a first face of the lens body adjacent an object and a second face of the lens body adjacent an image pickup surface is a Fresnel face.
According to a second aspect and feature of the present invention, in addition to the first feature, the lens body is comprised of a single lens piece.
With such configuration of the above feature, at least one of the faces of the lens body is formed into an aspherical shape, and at least one of the first face of the lens body adjacent the object and the second face of the lens body adjacent the image pickup surface is the Fresnel face. Therefore, the Petzval sum can be reduced to reduce the image surface curvature, thereby providing a good image surface. In addition, various aberrations such as a spherical aberration and the like can be corrected appropriately and hence, the optical characteristics of the image pickup lens can be enhanced remarkably.
According to a third aspect and feature of the present invention, in addition to the first feature, the lens body is formed of a first lens piece and a second lens piece bonded to each other.
With such configuration of the third feature, the lens body is formed of the first and second lens pieces bonded to each other. Therefore, various aberrations, particularly, the chromatic aberration can be corrected satisfactorily and hence, the optical characteristics of the image pickup lens can be enhanced.
The above and other objects, features and advantages of the invention will become apparent from the following description of the preferred embodiment taken in conjunction with the accompanying drawings.